It is common to many industrial ducting application to conduct hot carbon monoxide mixtures using a duct having an outer sheath tube lined with a thermally insulating composition and an inner pipe or tube for protecting the composition against erosion by the hot gas mixture transversing the duct.
Carbon-monoxide-containing hot gas mixtures for which the method and apparatus of the invention are intended, are found in systems for producing and processing or utilizing raw synthesis gases whose main components are hydrogen and carbon monoxide. Such gases can be produced, for example, by cracking of natural gas, by gasification of liquid hydrocarbons or by the gasification of coal. Process and reactors for these purposes can include those described in Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, Volume A 12, Pages 192 to 208.
They include also the tube furnace disclosed in detail in German Patent document DE-A 43 27 176.
The protective tube which is provided inwardly of the thermally insulating composition is divided into sections so that length changes of this tube resulting from temperature fluctuations can take place without restraint and therefore, without damage to the thermally insulating lining. The latter may be a tamping mass or other packing material as is used for the lining of metallurgical vessels, e.g. a alumina, silica or magnesia mass. The ability of the sections to expand and contract prevents binding or destruction of the internal protective tubing. The successive sections of this tubing are generally not mechanically fixed together to permit the expansion in length and, of course, contraction. The result is a butt joint which may permit small streams of the carbon monoxide containing gas to migrate outwardly into and through the protective composition as far as to the inner surface of the sheath tube.
After long operating periods, this leakage of the hot carbon-monoxide-containing gas may produce hot spots of the sheath tube.
Furthermore since the sheath tube is generally made of steel and steel is attacked by carbon monoxide, the sheath tube can be weakened and destroyed by the resulting formation of carbides. The formation of such carbides, referred to in the trade as metal dusting, is a recognized problem with duct work for such gas compositions.
The thermally insulating protective composition, which is a commercially available ramming or tamping compound or refractory lining material is usually porous so that the lining can allow the carbon-monoxide-containing hot gas mixture to pass to the inner surface of the sheath tube at least by diffusion.
The carbon-monoxide-containing hot gas mixture is, as has been noted, at a temperature of at least 500.degree. C. and in most cases at a temperature ranging from 600 to 1500.degree. C. The gas mixture usually has a CO content of at least 50% by volume.
Upon contact: of the CO with the metal, particularly steel at these temperatures, the reaction 2CO CO.sub.2 +C can occur with the C atom acting as a radical causing carbide formation in the metal. It is this carbide formation that is referred to as metal dusting corrosion and takes place commonly at temperatures in the range of 500.degree. C. to 850.degree. C.